Power transfer device with hydraulically-actuated clutch assembly

ABSTRACT

A torque transfer mechanism having a transfer clutch connecting a pair of rotary members and a electrohydraulic clutch actuator for controlling engagement of the transfer clutch. The clutch actuator includes a hydraulic pump and a hydraulically-actuated rotary operator. The hydraulic pump draws low pressure fluid from a sump and delivers high pressure fluid to a series of actuation chambers defined between coaxially aligned first and second components of the rotary operator. The magnitude of the fluid pressure delivered to the actuation chamber controls angular movement of the second component relative to the first component for energizing a pilot ball ramp unit. The pilot ball ramp mechanism applies a clutch actuation force on a pilot friction clutch which energized a main ball ramp unit for applying a clutch engagement force to a main friction clutch. A hydraulic control system adaptively regulates the fluid pressure delivered to the actuation chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/769,646 filed Jan. 30, 2004.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems forcontrolling the distribution of drive torque between the front and reardrivelines of a four-wheel drive vehicle and/or the left and rightwheels of an axle assembly. More particularly, the present invention isdirected to a power transmission device for use in motor vehicledriveline applications having a torque transfer mechanism equipped witha power-operated clutch actuator that is operable for controllingactuation of a multi-plate friction clutch.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora ofpower transfer systems are currently being incorporated into vehiculardriveline applications for transferring drive torque to the wheels. Inmany vehicles, a power transmission device is operably installed betweenthe primary and secondary drivelines. Such power transmission devicesare typically equipped with a torque transfer mechanism for selectivelyand/or automatically transferring drive torque from the primarydriveline to the secondary driveline to establish a four-wheel drivemode of operation. For example, the torque transfer mechanism caninclude a dog-type lock-up clutch that can be selectively engaged forrigidly coupling the secondary driveline to the primary driveline toestablish a “part-time” four-wheel drive mode. When the lock-up clutchis released, drive torque is only delivered to the primary driveline forestablishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the powertransmission device with an adaptively controlled transfer clutch inplace of the lock-up clutch. The transfer clutch is operable forautomatically directing drive torque to the secondary wheels, withoutany input or action on the part of the vehicle operator, when tractionis lost at the primary wheels for establishing an “on-demand” four-wheeldrive mode. Typically, the transfer clutch includes a multi-plate clutchassembly that is installed between the primary and secondary drivelinesand a clutch actuator for generating a clutch engagement force that isapplied to the clutch assembly. The clutch actuator can be apower-operated device that is actuated in response to electric controlsignals sent from an electronic controller unit (ECU). The electriccontrol signals are typically based on changes in current operatingcharacteristics of the vehicle (i.e., vehicle speed, interaxle speeddifference, acceleration, steering angle, etc.) as detected by varioussensors. Thus, such “on-demand” transfer clutch can utilize adaptivecontrol schemes for automatically controlling torque distribution duringall types of driving and road conditions. Such adaptively controlledtransfer clutches can also be used in association with a centerdifferential operably installed between the primary and secondarydrivelines for automatically controlling interaxle slip and torquebiasing in a full-time four-wheel drive application.

A large number of adaptively controlled transfer clutches have beendeveloped with an electro-mechanical clutch actuator that can regulatethe amount of drive torque transferred to the secondary driveline as afunction of the electric control signal applied thereto. In someapplications, the transfer clutch employs an electromagnetic clutch asthe power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024discloses an electromagnetic coil that is incrementally activated tocontrol movement of a ball-ramp drive assembly for applying a clutchengagement force to the multi-plate clutch assembly. Likewise, JapaneseLaid-open Patent Application No. 62-18117 discloses a transfer clutchequipped with an electromagnetic clutch actuator for directlycontrolling actuation of the multi-plate clutch pack assembly. Also,U.S. Pat. No. 6,158,561 discloses use of an electromagnetic actuator forengaging a pilot clutch which, in turn, controls energization of a ballramp unit for engaging the main clutch.

As an alternative to such electromagnetic clutch actuation systems, thetransfer clutch can employ an electric motor and a mechanical driveassembly as the power-operated clutch actuator. For example, U.S. Pat.No. 5,323,871 discloses a transfer clutch equipped with an electricmotor that controls rotation of a sector plate which, in turn, controlspivotal movement of a lever arm that is operable for applying the clutchengagement force to the multi-plate clutch assembly. Likewise, JapaneseLaid-open Patent Application No. 63-66927 discloses a transfer clutchwhich uses an electric motor to rotate one cam plate of a ball-rampoperator for engaging the multi-plate clutch assembly. Finally, U.S.Pat. Nos. 4,895,236 and 5,423,235 respectively disclose a transferclutch having an electric motor which drives a reduction gearset forcontrolling movement of a ball screw operator and a ball-ramp operatorwhich, in turn, apply the clutch engagement force to the clutchassembly.

In contrast to the electro-mechanical clutch actuators discussed above,it is also well known to equip the transfer clutch with anelectro-hydraulic clutch actuator. For example, U.S. Pat. Nos. 4,862,769and 5,224,906 generally disclose use of an electric motor or solenoid tocontrol the fluid pressure exerted by an apply piston on a multi-plateclutch assembly. In addition, U.S. Pat. No. 6,520,880 discloses ahydraulic actuation system for controlling the fluid pressure suppliedto a hydraulic motor arranged which is associated with a differentialgear mechanism in a drive axle assembly.

While many adaptive clutch actuation systems similar to those describedabove are currently used in four-wheel drive vehicles, a need exists toadvance the technology and address recognized system limitations. Forexample, the size and weight of the friction clutch components and theelectrical power requirements of the clutch actuator needed to providethe large clutch engagement loads make many systems cost prohibitive foruse in most four-wheel drive vehicle applications. In an effort toaddress these concerns, new technologies are being developed for use inpower-operated clutch actuator applications.

SUMMARY OF THE INVENTION

Thus, its is an objective of the present invention to provide a powertransmission device for use in a motor vehicle having a torque transfermechanism equipped with a unique power-operated clutch actuator that isoperable to control engagement of a multi-plate clutch assembly.

As a related objective of the present invention, the torque transfermechanism is well-suited for use in motor vehicle driveline applicationsto control the transfer of drive torque between first and second rotarymembers.

According to each preferred embodiment of the present invention, atorque transfer mechanism and an electrohydraulic control system aredisclosed for adaptively controlling the transfer of drive torquebetween first and second rotary members in a power transmission deviceof the type used in motor vehicle driveline applications. The torquetransfer mechanism includes a main clutch assembly operably disposedbetween the first and second rotary members, a pilot clutch assembly,and a rotary clutch operator. The rotary clutch operator includes afirst and second coaxially aligned components defining a plurality ofactuation chambers therebetween. During operation, the magnitude of thefluid pressure delivered by the hydraulic control system to theactuation chambers controls angular movement of the second componentrelative to the first component. Such relative angular movement controlsenergization of the pilot clutch assembly which, in turn, controls themagnitude of the compressive clutch engagement force applied to the mainclutch assembly, thereby controlling the drive torque transferred fromthe first rotary member to the second rotary member.

According to another feature of the present invention, theelectrohydraulic control system includes a fluid pump, an electric motorfor driving the pump, vehicle sensors for detecting various operatingcharacteristics of the motor vehicle, and an electronic control unit(ECU) for receiving input signals from the vehicle sensors andcontrolling energization of the electric motor. The ECU further controlsactuation of a control valve for controlling the magnitude of the fluidpressure supplied to the actuation chambers. A pressure sensor providesa pressure signal to the ECU that is indicative of the fluid pressure inthe actuation chambers.

The torque transfer mechanism of the present invention is adapted foruse in a power transmission device for adaptively controlling the drivetorque transferred between a primary driveline and a secondarydriveline. According to one preferred application, the powertransmission device of the present invention is a transfer case with thetorque transfer mechanism arranged as a torque transfer coupling forproviding on-demand torque transfer from the primary driveline to thesecondary driveline. In a related application, the torque transfermechanism is arranged as a torque bias coupling for varying the torquedistribution and limiting interaxle slip between the primary andsecondary driveline. According to another preferred application, thepower transmission device is a drive axle assembly with the torquetransfer mechanism arranged as a torque bias coupling to control speeddifferentiation and torque distribution across a differential unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent to those skilled in the art from analysis of thefollowing written description, the appended claims, and accompanyingdrawings in which:

FIG. 1 illustrates the drivetrain of a four-wheel drive vehicle equippedwith a power transmission device according to the present invention;

FIG. 2 is a sectional view of a transfer case associated with thedrivetrain shown in FIG. 1 and which is equipped with a torque transfermechanism according to a preferred embodiment of the present invention;

FIGS. 3A and 3B are enlarged partial views taken from FIG. 2 showingcomponents of the torque transfer mechanism is greater detail;

FIG. 4 is a partial sectional view of a rotary operator mechanismassociated with the torque transfer mechanism of the present invention;

FIG. 5 is a schematic diagram of a hydraulic control circuit associatedwith the torque transfer mechanism of the present invention;

FIG. 6 is a schematic illustration of an alternative driveline for afour-wheel drive motor vehicle equipped with a power transmission deviceof the present invention;

FIG. 7 is a schematic illustration of a drive axle assembly associatedwith the drivetrain shown in FIG. 6 and equipped with a torque transfermechanism according to the present invention;

FIG. 8 is a schematic illustration of an alternative drive axle assemblyoperable for use with either of the drivetrain shown in FIGS. 1 and 6;

FIG. 9 is a schematic illustration of another alternative embodiment ofa power transmission device according to the present invention;

FIG. 10 illustrates another alternative drivetrain arrangement for afour-wheel drive motor vehicle equipped with another power transmissiondevice embodying the present invention;

FIGS. 11 through 14 schematically illustrate different embodiments ofthe power transmission device shown in FIG. 10; and

FIG. 15 is a schematic illustration of an alternative construction forthe power transmission device shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism thatcan be adaptively controlled for modulating the torque transferred froma first rotary member to a second rotary member. The torque transfermechanism finds particular application in power transmission devices foruse in motor vehicle drivelines such as, for example, a torque transferclutch in a transfer case, a power take-off unit or an in-line torquecoupling, a torque biasing clutch associated with a differential unit infull-time transfer cases or power take-off units or in a drive axleassembly, or any other possible torque transfer application. Thus, whilethe present invention is hereinafter described in association withparticular power transmission devices for use in specific drivelineapplications, it will be understood that the arrangements shown anddescribed are merely intended to illustrate embodiments of the presentinvention.

With particular reference to FIG. 1 of the drawings, a drivetrain 10 fora four-wheel drive vehicle is shown. Drivetrain 10 includes a primarydriveline 12, a secondary driveline 14, and a powertrain 16 fordelivering rotary tractive power (i.e., drive torque) to the drivelines.In the particular arrangement shown, primary driveline 12 is the reardriveline while secondary driveline 14 is the front driveline.Powertrain 16 includes an engine 18, a multi-speed transmission 20, anda power transmission device hereinafter referred to as transfer case 22.Rear driveline 12 includes a pair of rear wheels 24 connected atopposite ends of a rear axle assembly 26 having a rear differential 28coupled to one end of a rear prop shaft 30, the opposite end of which iscoupled to a rear output shaft 32 of transfer case 22. Likewise, frontdriveline 14 includes a pair of front wheels 34 connected at oppositeends of a front axle assembly 36 having a front differential 38 coupledto one end of a front prop shaft 40, the opposite end of which iscoupled to a front prop shaft 42 of transfer case 22.

With continued reference to the drawings, drivetrain 10 is shown tofurther include an electronically-controlled power transfer system 44for permitting a vehicle operator to select between a two-wheel drivemode, a locked (“part-time”) four-wheel drive mode, and an adaptive(“on-demand”) four-wheel drive mode. In this regard, transfer case 22 isequipped with a transfer clutch 50 that can be selectively actuated fortransferring drive torque from rear output shaft 32 to front outputshaft 42 for establishing both of the part-time and on-demand four-wheeldrive modes. Power transfer system 44 further includes anelectrohydraulic clutch actuator 52 for actuating transfer clutch 50,vehicle sensors 54 for detecting certain dynamic and operationalcharacteristics of the motor vehicle, a mode select mechanism 56 forpermitting the vehicle operator to select one of the available drivemodes, and an electronic control unit (ECU) 58 for controlling actuationof clutch actuator 52 in response to input signals from vehicle sensors54 and mode selector 56.

Transfer case 22 is shown in FIG. 2 to include a multi-piece housing 60from which rear output shaft 32 is rotatably supported by a pair oflaterally-spaced bearing assemblies 62. Rear output shaft 32 includes aninternally-splined first end segment 64 adapted for connection to theoutput shaft of transmission 20 and a yoke assembly 66 secured to itssecond end segment 68 that is adapted for connection to rear propshaft30. Front output shaft 42 is likewise rotatably supported from housing60 by a pair of laterally-spaced bearing assemblies 70 and includes ayoke-type end segment 72 that is adapted for connection to frontpropshaft 40.

In general, transfer clutch 50 and electrohydraulic clutch actuator 52define a torque transfer mechanism according to the preferred embodimentof the present invention. Transfer clutch 50 includes a main clutchassembly 74 and a pilot clutch assembly 76. Main clutch assembly 74 isshown to include a first multi-plate friction clutch 78 and a first ballramp unit 80. Likewise, pilot clutch assembly 76 is shown to include asecond multi-plate friction clutch 82 and a second ball ramp unit 84.First friction clutch 78 includes a hub 86 fixed (i.e., splined) forrotation with rear output shaft 32, a drum 88 and a multi-plate clutchpack 90 that is operably disposed between hub 84 and drum 88. Clutchpack 90 includes a set of outer clutch plates 92 splined for rotationwith drum 88 and which are interleaved with a set of inner clutch plates94 splined for rotation with hub 84. As will be detailed, clutchactuator 52 is operable for causing a compressive clutch engagementforce to be exerted on clutch pack 90. Such engagement of clutch pack 90causes rotary power (“drive torque”) to be transferred from rear outputshaft 32 to front output shaft 42 through a transfer assembly 96.Transfer assembly 96 includes a first sprocket 98 fixed (i.e., splined)for rotation with drum 88, a second sprocket 100 fixed (i.e., splined)for rotation with front output shaft 42, and a power chain 102encircling sprockets 98 and 100. First sprocket 98 is shown fixed to anend plate segment 104 of drum 88 and is rotatably supported on rearoutput shaft 32 via a suitable bearing assembly 106. A thrust bearing108 is shown disposed between first sprocket 102 and a lock ring 109fixed to rear output shaft 32.

First ball ramp unit 80 includes a first cam member 110, a second cammember 112 and rollers 114. First cam member 110 is splined for commonrotation with drum 88 and bi-directional translational movement relativeto clutch pack 90. Specifically, first cam member 110 is axiallymoveable between a first or “released” position and a second or “locked”position. In its released position, first cam member exerts a minimumclutch engagement force on clutch pack 90 such that virtually no drivetorque is transferred from rear output shaft 32 to front output shaft42, thereby establishing the two-wheel drive mode. In contrast, movementof first cam member 110 to its locked position causes a maximum clutchengagement force to be exerted on clutch pack 90 such that front outputshaft 42 is, in effect, coupled for common rotation with rear outputshaft 32, thereby establishing the part-time four-wheel drive mode.Accordingly, variable control of the movement of first cam member 110between its released and locked position results in adaptive regulationof the drive torque transferred to front output shaft 42, therebyestablishing the on-demand four-wheel drive mode.

Second cam member 112 of first ball ramp unit 80 is operably connectedto second friction clutch 82. In addition, rollers 114 are disposed in acam channel defined between cam tracks 116 formed in first cam member110 and cam tracks 118 formed in second cam member 112. Preferably, aplurality of such cam channels are provided and which are eachconfigured as an oblique section of a helical torus. Balls 114 and camtracks 116,118 may be replaced with alternative components and/ortapered ramp profiles that functions to cause axial movement of firstcam member 110 in response to relative angular movement between the cammembers. In any arrangement, the load transferring components can not beself-locking or self-engaging so as to permit fine control over thetranslational movement of first cam member 110 for providing precisecontrol of the clutch engagement force applied to clutch pack 90. Athrust bearing assembly 120 is disposed between second cam member 112and a retainer plate 122 that is splined to drum 88. A lock ring 124axially locates retainer plate 122 for preventing axial movement ofsecond cam member 112.

Second friction clutch 82 includes a multi-plate clutch pack 128 that isoperably disposed between second cam member 112 of first ball ramp unit80 and hub 86 of first friction clutch 78. Clutch pack 128 includes aset of outer plates 130 splined for rotation with second cam member 112and which are interleaved with a set of inner clutch plates 132 splinedfor rotation with hub 86. Second ball ramp unit 84 includes a first camring 134, a second cam ring 136, and rollers 138. First cam ring 134 isfixed to a pressure plate 140 which, in turn, is splined for rotationwith hub 86 of first friction clutch 78. Rollers 138 are disposed in camchannels defined between cam tracks 142 formed in first cam ring 136 andcam tracks 144 formed in second cam ring 136. Again, it is preferredthat a plurality of such cam channels be provided between the cam ringswith each being configured as an oblique section of a torus.Furthermore, second ball ramp unit 84 is also not self-locking orself-engaging to permit accurate control of bi-directional translationalmovement of pressure plate 140 relative to clutch pack 128 that iscaused in response to relative rotation between cam rings 134 and 136. Athrust bearing 146 is disposed between second cam ring 136 and aretainer ring 148 which, in turn, is rigidly secured for rotation withhub 86 via bolts 150. Such translational movement of pressure plate 140is operable for controlling the magnitude of pilot actuation forceexerted on clutch pack 128 which, in turn, controls energization offirst ball ramp unit 80. With pressure plate 140 in a first or“retracted” position, a minimum pilot actuation force is exerted onclutch pack 128 such that first and second cam members of first ballramp unit 80 are permitted to rotate together, thereby maintaining firstcam member 110 in its released position. In contrast, movement ofpressure plate 140 to a second or “extended” position causes a maximumpilot actuation force to be exerted on clutch pack 128 which, in turn,causes second cam member 112 to rotate relative to first cam member 110.Such relative rotation results in axial movement of first cam member 110to its locked position.

To provide means for moving pressure plate 140 between its retracted andextended positions, clutch actuator 52 generally includes a rotaryoperator 152 and an electrohydraulic power unit 154. Power unit 154 issecured to housing 60 and includes an electric motor 156 and a fluidpump 158. Rotary operator 152 is shown to include a first or “reaction”ring 160 that is concentrically aligned with a second or “actuator” ring162. The rings are retained between clutch hub 86 and retainer ring 148.As seen, bolts 150 also pass through mounting bores 164 in reaction ring160 such that reaction ring 160 is fixed to hub 86 for common rotationwith rear output shaft 32.

As best seen from FIG. 4, reaction ring 160 includes a cylindrical bodysegment 166 and a plurality of radially outwardly projecting lugs 168.Lugs 168 define a complementary number of longitudinally extendingchannels 170 therebetween with a like number of radial inlet ports 172extending through body segment 166 and communicating with channels 170.Actuator ring 162 also has a cylindrical body segment 174 that is fixedvia a spline connection 176 to second cam ring 136 of second ball rampunit 84. In addition, a plurality of radially projecting lugs 180 extendinwardly from body segment 174. Each lug 180 extends into acorresponding one of channels 170 so as to define a series of actuationchambers 182 delimited by a face surface 184 of lugs 168 and a facesurface 186 of lugs 180. Likewise, a distal end surface 188 on each lug168 is in sliding engagement with an inner wall surface 190 of bodysegment 174 while a distal end surface 192 on each lug 180 is in slidingengagement with outer wall surface 194 of body segment 166 so as tofurther delimit each actuation chamber 182.

As noted, reaction ring 160 includes a series of inlet ports 172 thatare in communication with actuation chambers 182. As will be described,increasing the fluid pressure delivered through inlet ports 172 toactuation chambers 182 causes actuator ring 162 to move (i.e., index) ina first rotary direction (i.e., clockwise) relative to reaction ring 160for energizing pilot ball ramp unit 84 which, in turn, causescorresponding movement of pressure plate 140 toward its extendedposition, In contrast, decreasing the fluid pressure in actuationchambers 182 causes actuator ring 162 to move in a second rotarydirection (i.e., counterclockwise) relative to reaction ring 160 forde-energizing pilot ball ramp unit 84 which, in turn, causescorresponding movement of pressure plate 140 toward its retractedposition.

Main ball ramp unit 80 further includes a torsional return spring 196that is operably connected between first cam member 110 and second cammember 112. Return spring 196 functions to angularly bias the cammembers for moving first cam member 110 toward its released position soas to de-energize main ball ramp unit 80. Such angular movement betweenthe cam members due to the biasing of return spring 196 also results inrearward translation of first cam ring 134 toward its retracted positionfor de-energizing pilot ball ramp unit 84. The resulting angularmovement of second cam ring 136 relative to first cam ring 134 acts toindex actuator ring 162 in the second direction relative to reactionring 160 toward a first or “low pressure” position, as is shown in FIG.4. Rotary operator 152 is designed to provide fluid leakage paths whichpermit fluid in actuation chambers 182 to leak out at a predeterminedrate so as to permit the biasing force of return spring 196 to angularlybias actuator ring 162 to move toward its low pressure position.

In operation, the delivery of fluid to actuation chambers 182 causesactuator ring 162 to rotate relative to reaction ring 160 in the firstdirection from its low pressure position toward a second or “highpressure” position which, in turn, results in corresponding relativerotation between cam rings 134 and 136 for moving first cam ring 134from its retracted position toward its extended position. In essence,such angular movement of actuator ring 162 acts to initiate energizationof pilot ball ramp unit 84. Accordingly, the profile of cam tracks 142and 144 establishes the resultant amount of forward axial movement offirst cam ring 134 which causes pressure plate 140 to exert acorresponding pilot actuation force on clutch pack 128. Engagement ofclutch pack 128 effectively couples second cam member 112 of main ballramp unit 80 for rotation with hub 86 and rear output shaft 32. Thisaction results in relative rotation between the cam members 110 and 112and translational movement of first cam member 110 toward its lockedposition.

With pressure plate 140 in its retracted position, first cam member 110is located in its released position such that virtually no drive torqueis transferred from rear output shaft 32 to front output shaft 42through transfer clutch 50, thereby effectively establishing thetwo-wheel drive mode. In contrast, movement of pressure plate 140 to itsextended position causes corresponding movement of member 110 to itslocked position. As such, a maximum amount of drive torque istransferred to front output shaft 42 for, in effect, coupling frontoutput shaft 42 for common rotation with rear output shaft 32, therebyestablishing the part-time four-wheel drive mode. Accordingly,controlling the position of pressure plate 140 between its retracted andextended positions permits variable control of the amount of drivetorque transferred from rear output shaft 32 to front output shaft 42,thereby establishing the on-demand four-wheel drive mode. Thus, themagnitude of the fluid pressure supplied to actuation chambers 182controls the angular position of actuator ring 162 relative to reactionring 160 for controlling actuation of pilot ball ramp unit 84 and, inturn, axial movement of pressure plate 120 between its retracted andextended positions.

A hydraulic flow circuit is provided within transfer case 22 forsupplying fluid from pump 158 to actuation chambers 182. Referringinitially to FIG. 5, a schematic of the hydraulic flow circuit will bedescribed. Specifically, hydraulic fluid from a source of fluid or sump200 maintained with transfer case housing 60 is drawn through a firstflow path 202 to an inlet of pump 158. Actuation of motor 156 controlsthe magnitude of the line pressure delivered through a second flow path204 from an outlet of pump 158 to an inlet of an electrically-actuatedcontrol valve 206. Control valve 206 includes a moveable valve element208 (see FIG. 3B) that regulates the delivery of fluid from its inlet toat least one of a pair of outlets. It will be understood that any typeof electrically-actuated control valve capable of regulating the fluidpressure supplied to actuation chambers 182 can be used. The firstoutlet supplies fluid to actuation chambers 182 of rotary operator 152through a third flow path 210 while the second outlet supplies fluid tocool and lubricate clutch pack 90 through a fourth flow path 212.

ECU 58 sends electrical control signals to both electric motor 156 andcontrol valve 206 for accurately controlling the fluid pressure suppliedthrough third flow path 210 to actuation chambers 182 by utilizing apredefined control strategy that is based on the mode signal from modeselector 56 and the sensor input signals from vehicle sensors 54. Apressure sensor 214 sends a signal to ECU 58 that is indicative of thefluid pressure in actuation chambers 182. In addition, a temperaturesensor 216 sends a signal to ECU 58 that is indicative of the fluidtemperature in fourth flow path 212 for permitting improved control overthe cooling of clutch pack 90. Finally, a pressure relief valve 218 isprovided for selectively venting fluid from actuation chambers 182 intofourth flow path 212 so as to limit the fluid pressure within actuationchambers 182 to a predetermined maximum pressure value.

Referring primarily now to FIGS. 3A and 3B, the structure associatedwith transfer case 22 for providing the flow paths schematically shownin FIG. 5 will now be described in greater detail. As seen, a centralchamber 220 is formed in rear output shaft 32 and is sealed via a sealcup 222. A separator 224 is retained within chamber 220 and includes acylindrical hub segment 226 and an elongated tube segment 228. Hubsegment 226 has a series of radial flow ports 230 which communicate witha large diameter longitudinal flow port 232 formed in tube segment 228.In addition, an end portion of tube segment 228 is retained in a smalldiameter portion of central chamber 220 and has a flange 234 delimitingan intermediate diameter portion of central chamber from its largediameter portion. A ring seal 236 provides a fluid-tight interfacebetween the intermediate and large diameter portions of central chamber220. In addition, one or more by-pass ports 238 extend through hubsegment 226 of separator 224. Suitable seal rings 240 provide afluid-tight seal between radial flow ports 230 and large diameterportion of chamber 220.

First flow path 202 includes an inlet hose 242 which provides fluidcommunication between the internal fluid sump 200 provided withinhousing 60 and the inlet to pump 158. Second flow path 204 includes aflow port 244 which delivers fluid at line pressure to a valve chamber246 within which valve element 208 is retained. Flow port 244 and valvechamber 246 are formed in a valvebody segment 60A of housing. Third flowpath 210 includes a flow passage 250 formed in valvebody housing segment60A which communicates with the first outlet of valve chamber 246, anannular chamber 252 which communicates with passage 250, and a series ofradial passages 254 formed in rear output shaft 32 which provide fluidcommunication between chamber 252 and the intermediate diameter portionof central chamber 220. Radial ports 230 and longitudinal port 232 inseparator 224 are also part of third flow path 210 and are in fluidcommunication with intermediate diameter portion of central chamber 220via one or more throughbores 256 in tube segment 228. Third flow path210 also includes a plurality of radial flow passages 258 formed throughrear output shaft 32 which connect radial ports 230 in separator 224with radial inlet ports 172 extending through body segment 166 ofreaction ring 160. As such, the fluid supplied from valve chamber 246 tothe inlet of flow passage 250 will flow through third flow path 210 intoactuation chambers 182.

Fourth flow path 212 includes a flow passage 270 in valvebody housingsegment 60A which communicates with the second outlet of valve chamber246, an annular chamber 272 which communicates with passage 270, and aseries of radial passages 274 formed in rear output shaft 32 whichinterconnect chamber 272 to a first chamber 276 formed in large diameterportion of central chamber 220. First chamber 276 surrounds tube segment288 of separator 224 and is in fluid communication with a second chamber278 via by-pass ports 238. Fourth flow path 212 further includes aseries of radial passages 280 formed through rear output shaft 32 whichcommunicate with throughbores 282 formed in clutch hub 86. As such, lowpressure fluid supplied from valve chamber 246 to the inlet of flowpassage 270 will flow through this flow path and through the interleavedclutch plates of clutch pack 90 before returning to sump 200. In thismanner, the heat generated within clutch pack 90 can be transferred tothe fluid prior to its return to sump 200.

In operation, if the two-wheel drive mode is selected, control valve 206is de-actuated such that valve element 208 moves to a position where theinlet to flow passage 250 is blocked. As such, the biasing of returnspring 196 and the leakage paths within rotary operator 152 causeactuator ring 162 to index relative to reaction ring 160 in the seconddirection toward its low pressure position, whereby pilot ball ramp unit84 is de-energized and pressure plate 140 is permitted to return to itsretracted position. With pilot clutch 82 released, main ball ramp unit80 is de-energized such that main clutch 78 is also released. Incontrast, upon selection of the part-time four-wheel drive mode, controlvalve 206 is fully activated to move valve element 208 to a positionwhere flow passage 250 receives the full line pressure from pump 158through port 244. This high pressure fluid is delivered through thirdflow path 210 to actuation chambers 182 for causing actuator ring 162 toindex relative to reaction ring 160 in the first direction until locatedin its high pressure position, whereby pilot ball ramp unit 84 is fullyenergized and pressure plate 140 is moved to its extended position forfully engaging pilot clutch 82. As such, main ball ramp unit 80 isenergized to move first cam member 110 to its locked position for fullyengaging main friction clutch 78. As stated, the pressure signal sentfrom pressure sensor 214 to ECU 58 in conjunction with the use ofpressure relief valve 218 function to limit the maximum fluid pressurethat can be delivered to actuation chambers 182, thereby preventingdamage to clutch pack 90.

When mode selector 52 indicates selection of the on-demand four-wheeldrive mode, ECU 58 energizes motor 156 for initiating a fluid pumpingaction in pump 158 and also controls energization of control valve 206for supplying a predetermined initial fluid pressure to actuationchambers 182 that results in a slight indexing of actuator ring 162relative to reaction ring 160 in the first direction. This angularmovement causes actuator ring 162 to move from its low pressure positionto an intermediate or “ready” position which, in turn, results in mainball ramp unit 80 moving first cam member 110 from its released positionto a “stand-by” position. Accordingly, a predetermined minimum amount ofdrive torque is delivered to front output shaft 42 through transferclutch 50 in this adapt-ready condition. Thereafter, ECU 58 determineswhen and how much drive torque needs to be transferred to front outputshaft 42 based on the current tractive conditions and/or operatingcharacteristics of the motor vehicle, as detected by sensors 54. Sensors54 detect such parameters as, for example, the rotary speed of theoutput shafts, the vehicle speed and/or acceleration, the transmissiongear, the on/off status of the brakes, the steering angle, the roadconditions, etc. Such sensor signals are used by ECU 58 to determine adesired output torque value utilizing a control scheme that isincorporated into ECU 58. This desired torque value is used to activelycontrol actuation of electric motor 156 and control valve 206 togenerate a corresponding pressure level in actuation chamber 182. Onenon-limiting example of a clutch control scheme and the various sensorsused therewith is disclosed in commonly-owned U.S. Pat. No. 5,323,871which is incorporated by reference herein.

In addition to adaptive torque control, the present invention permitsautomatic release of transfer clutch 50 in the event of an ABS brakingcondition or during the occurrence of an over-temperature condition.Furthermore, while the control scheme was described based on anon-demand strategy, it is contemplated that a differential or “mimic”control strategy could likewise be used. Specifically, the torquedistribution between rear output shaft 32 and front output shaft 42 canbe controlled to maintain a predetermined rear/front ratio (i.e., 70:30,50:50, etc.) so as to simulate the inter-axle torque splitting featuretypically provided by a mechanical differential unit. Regardless of thecontrol strategy used, accurate control of the fluid pressure deliveredfrom pump 156 to actuation chambers 182 of rotary operator 152 willresult in the desired torque transfer characteristics across transferclutch 50. Furthermore, it should be understood that mode selectmechanism 56 could also be arranged to permit selection of only twodifferent drive modes, namely the on-demand 4WD mode and the part-time4WD mode. Alternatively, mode select mechanism 56 could be eliminatedsuch that the on-demand 4WD mode is always operating in a manner that istransparent to the vehicle operator.

To illustrate an alternative power transmission device to which thepresent invention is applicable, FIG. 6 schematically depicts afront-wheel based four-wheel drivetrain layout 10′ for a motor vehicle.In particular, engine 18 drives a multi-speed transmission 20′ having anintegrated front differential unit 38′ for driving front wheels 34 viaaxle shafts 33. A transfer or power take-off unit (PTU) 300 is alsodriven by transmission 20′ for delivering drive torque to the inputmember of a torque transfer mechanism, such as an in-line torquetransfer coupling 302, via a drive shaft 30′. Torque transfer coupling302 is preferably integrated with the components of conventional axleassembly 26 to define a drive axle assembly 26′. In particular, theinput member of torque coupling 302 is coupled to drive shaft 30′ whileits output member is coupled to a drive component of rear differential28 which, in turn, drives rear wheels 24 via axleshafts 25. Accordingly,when sensors 54 indicate the occurrence of a front wheel slip condition,ECU 58 adaptively controls actuation of torque coupling 302 such thatdrive torque is delivered “on-demand” to rear wheels 24. It iscontemplated that torque transfer coupling 302 includes a transferclutch and an electrohydraulic clutch actuator that are similar in bothstructure and function to the torque transfer mechanism previouslydescribed herein. Accordingly, common reference numerals will be usedhereinafter to identify components previously described.

Referring to FIG. 7, torque coupling 302 is schematically illustrated tobe operably disposed between drive shaft 30′ and rear differential 28.Rear differential 28 includes a pair of side gears 304 that areconnected to rear wheels 24 via rear axle shafts 25. Differential 28also includes pinions 306 that are rotatably supported on pinion shaftsfixed to a carrier 308 and which mesh with side gears 304. Aright-angled drive mechanism is associated with differential 28 andincludes a ring gear 310 that is fixed for rotation with carrier 308 andmeshed with a pinion gear 312 that is fixed for rotation with a pinionshaft 314. Torque coupling 302 is schematically shown to include variouscomponents of transfer clutch 50 that are operably disposed betweendriveshaft 30′ and pinion shaft 314. In particular, transfer clutch 50is schematically shown to include main friction clutch 78 and main ballramp unit 80 as well as pilot friction clutch 82 and pilot ball rampunit 84. Torque coupling 302 also is shown schematically to includeclutch actuator 52 that can be adaptively actuated for controlling themagnitude of the clutch engagement force applied to transfer clutch 50and thus the amount of drive torque transferred from drive shaft 30′ torear differential 28. Actuator 52 includes rotary operator 152 and theelectrohydraulic control system previously disclosed in FIG. 5 foradaptively controlling actuation of rotary operator 152. In this regard,power transfer system 44 is illustrated in block format and iscontemplated to include all electrical and hydraulic components andsub-systems required to adaptively control actuation of rotary operator152.

Torque coupling 302 permits operation in any of the drive modespreviously disclosed. For example, if the on-demand 4WD mode isselected, ECU 58 regulates activation of clutch actuator 52 in responseto the operating conditions detected by sensors 54 by controllablyvarying the electric control signal sent motor 128 and control valve206. Selection of the part-time 4WD mode results in complete engagementof main clutch pack 90 such that pinion shaft 314 is, in effect, rigidlycoupled to driveshaft 30′. Finally, in the two-wheel drive mode, mainclutch pack 90 is released such that pinion shaft 312 is free to rotaterelative to driveshaft 30′. Alternatively, elimination of mode selectmechanism 56 would provide automatic adaptive operation of torquecoupling 302.

The arrangement shown for drive axle assembly 26′ of FIG. 7 is operableto provide on-demand four-wheel drive by adaptively controlling thetransfer of drive torque from the primary driveline to the secondarydriveline. In contrast, a drive axle assembly 320 is shown in FIG. 8wherein torque coupling 302 is now operably installed betweendifferential case 308 and one of axleshafts 25 to provide an adaptive“side-to-side” torque biasing and slip limiting feature. As before,torque coupling 302 is schematically shown to include a transfer clutch50 and an electrohydraulic clutch actuator 52, the construction andfunction of which are understood to be similar to the detaileddescription previously provided herein for each sub-assembly.

Referring now to FIG. 9, a drive axle assembly 322 is schematicallyshown to include a pair of torque couplings 302L and 302R that areoperably installed between a driven pinion shaft 314 or 30′ andaxleshafts 25. The driven pinion shaft drives a right-angled gearsetincluding pinion 312 and ring gear 310 which, in turn, drives a transfershaft 324. First torque coupling 302L is shown disposed between transfershaft 324 and the left one of axleshafts 25 while second torque coupling302R is disposed between transfer shaft 324 and the right axle shaft 25.Each torque coupling includes a corresponding transfer clutch 50L, 50Rand electrohydraulic clutch actuator 52L, 52R. Accordingly, independenttorque transfer and slip control is provided between the driven pinionshaft and each rear wheel 24 pursuant to this arrangement.

To illustrate additional alternative power transmission devices to whichthe present invention is applicable, FIG. 10 schematically depicts afront-wheel based four-wheel drive drivetrain layout 10″ for a motorvehicle. In particular, engine 18 drives multi-speed transaxle 20′ whichhas an integrated front differential unit 38′ for driving front wheels34 via axle shafts 33. As before, PTU 300 is also driven by transaxle20′ for delivering drive torque to the input member of a torque transfercoupling 330. The output member of torque transfer coupling 330 iscoupled to propshaft 30′ which, in turn, drives rear wheels 24 viaaxleshafts 25. Rear axle assembly 26 can be a traditional driven axlewith a differential or, in the alternative, be similar to the drive axlearrangements described in regard to FIG. 8 or 9. Accordingly, inresponse to detection of certain vehicle characteristics by sensors 54(i.e., the occurrence of a front wheel slip condition), power transfersystem 44 causes torque coupling 330 to deliver drive torque “on-demand”to rear wheels 24. It is contemplated that torque coupling 330 would begenerally similar in structure and function to that of torque transfercoupling 302 previously described herein. As such, its primarycomponents of transfer clutch 50 and electrohydraulic clutch actuator 52are again schematically shown.

Referring now to FIG. 11, torque coupling 330 is schematicallyillustrated in association with an on-demand four-wheel drive systembased on a front-wheel drive vehicle similar to that shown in FIG. 10.In particular, an output shaft 332 of transaxle 20′ is shown to drive anoutput gear 334 which, in turn, drives an input gear 336 that is fixedto a carrier 338 associated with front differential unit 38′. To providedrive torque to front wheels 34, front differential unit 38′ includes apair of side gears 340 that are connected to front wheels 34 viaaxleshafts 33. Differential unit 38′ also includes pinions 342 that arerotatably supported on pinion shafts fixed to carrier 338 and which aremeshed with side gears 340. A transfer shaft 344 is provided fortransferring drive torque from carrier 338 to a clutch hub 84 associatedwith transfer clutch 50. PTU 300 is a right-angled drive mechanismincluding a ring gear 346 fixed for rotation with drum 88 of transferclutch 50 and which is meshed with a pinion gear 348 fixed for rotationwith propshaft 30′. According to the present invention, the componentsschematically shown for torque transfer coupling 330 are understood tobe similar to those previously described. In operation, drive torque isadaptively transferred on-demand from the primary (i.e., front)driveline to the secondary (i.e., rear) driveline.

Referring to FIG. 12, a modified version of the power transmissiondevice shown in FIG. 11 is now shown to include a second torque coupling330A that is arranged to provide a limited slip feature in associationwith primary differential 38′. As before, adaptive control of torquecoupling 330 provides on-demand transfer of drive torque from theprimary driveline to the secondary driveline. In addition, adaptivecontrol of second torque coupling 330A provides on-demand torque biasing(side-to-side) between axleshafts 33 of primary driveline 14.

FIG. 13 illustrates another modified version of FIG. 9 wherein anon-demand four-wheel drive system is shown based on a rear-wheel drivemotor vehicle that is arranged to normally deliver drive torque to rearwheels 24 while selectively transmitting drive torque to front wheels 34through a torque coupling 330. In this arrangement, drive torque istransmitted directly from transmission output shaft 332 to powertransfer unit 300 via a drive shaft 350 which interconnects input gear336 to ring gear 346. To provide drive torque to front wheels 34, torquecoupling 330 is shown operably disposed between drive shaft 350 andtransfer shaft 344. In particular, transfer clutch 50 is arranged suchthat drum 88 is driven with ring gear 346 by drive shaft 350. As such,clutch actuator 52 functions to transfer drive torque from drum 88through clutch pack 90 to hub 84 which, in turn, drives carrier 338 ofdifferential unit 38′ via transfer shaft 344.

In addition to the on-demand four-wheel drive systems shown previously,the power transmission technology of the present invention can likewisebe used in full-time four-wheel drive systems to adaptively bias thetorque distribution transmitted by a center or “interaxle” differentialunit to the front and rear drivelines. For example, FIG. 14schematically illustrates a full-time four-wheel drive system which isgenerally similar to the on-demand four-wheel drive system shown in FIG.13 with the exception that an interaxle differential unit 360 is nowoperably installed between carrier 338 of front differential unit 38′and transfer shaft 344. In particular, output gear 336 is fixed forrotation with a carrier 362 of interaxle differential 360 from whichpinion gears 364 are rotatably supported. A first side gear 366 ismeshed with pinion gears 364 and is fixed for rotation with drive shaft350 so as to be drivingly interconnected to the rear driveline throughpower transfer unit 300. Likewise, a second side gear 368 is meshed withpinion gears 364 and is fixed for rotation with carrier 338 of frontdifferential unit 38′ so as to be drivingly interconnected to the frontdriveline. Torque coupling 330 is now shown to be operably disposedbetween side gears 366 and 368. Torque coupling 330 is operably arrangedbetween the driven outputs of interaxle differential 360 for providingan adaptive torque biasing and slip limiting function between the frontand rear drivelines.

Referring now to FIG. 15, a full-time 4WD system is shown to include atransfer case 22′ which is generally similar to transfer case 22 of FIG.2 except that an interaxle differential 380 is provided between an inputshaft 382 and output shafts 32 and 42. As is conventional, input shaft382 is driven by the output of transmission 20. Differential 380includes an input defined as a planet carrier 384, a first outputdefined as a first sun gear 386, a second output defined as a second sungear 388, and a gearset for permitting speed differentiation betweenfirst and second sun gears 386 and 388. The gearset includes a pluralityof meshed pairs of first planet gears 390 and second pinions 392 whichare rotatably supported by carrier 384. First planet gears 390 are shownto mesh with first sun gear 386 while second planet gears 392 are meshedwith second sun gear 388. First sun gear 386 is fixed for rotation withrear output shaft 32 so as to transmit drive torque to the reardriveline. To transmit drive torque to the front driveline, second sungear 388 is coupled to transfer assembly 100 which again includes firstsprocket 102 rotatably supported on rear output shaft 32, secondsprocket 106 fixed to front output shaft 42, and power chain 110.

A number of preferred embodiments have been disclosed to provide thoseskilled in the art an understanding of the best mode currentlycontemplated for the operation and construction of the presentinvention. The invention being thus described, it will be obvious thatvarious modifications can be made without departing from the true spiritand scope of the invention, and all such modifications as would beconsidered by those skilled in the art are intended to be includedwithin the scope of the following claims.

1. A power transmission device comprising: a first rotary member; asecond rotary member; a torque transfer mechanism operable fortransferring drive torque between said first and second members, saidtorque transfer mechanism having a transfer clutch disposed between saidfirst and second rotary members and a rotary operator for applying aclutch engagement force to said transfer clutch, said rotary operatorincluding a first component rotatably driven by one of said first andsecond rotary members and a second component coaxially aligned with saidfirst component so as to define a plurality of actuation chamberstherebetween, said second component is adapted to rotate relative tosaid first component in response to fluid pressure in said actuationchambers for generating said clutch engagement force; and a hydrauliccontrol system for regulating the fluid pressure supplied to saidactuation chambers.
 2. The power transmission device of claim 1 whereinsaid hydraulic control system includes a pump, a motor driving said pumpand a control valve disposed in a hydraulic circuit between said pumpand said actuation chambers for regulating the fluid pressure suppliedto said actuation chambers.
 3. The power transmission device of claim 2wherein angular movement of said second component to a low pressureposition relative to said first component causes a minimum clutchactuation force to be applied to said transfer clutch, wherein angularmovement of said second component to a high pressure position relativeto said first component causes a maximum clutch actuation force to beapplied to said transfer clutch, and wherein said second component ismoveable between its low pressure and high pressure positions due to themagnitude of the fluid pressure delivered from said pump through saidcontrol valve to said actuation chambers.
 4. The power transmissiondevice of claim 1 wherein said first component of said rotary operatoris a first ring having a plurality of first lugs so as to define aplurality of channels therebetween, and wherein said second component ofsaid rotary actuator is an a second ring having a plurality of secondlugs which extend into said channels so as to define a series of saidactuation chambers between adjacent pairs of said first and second lugs.5. The power transmission device of claim 4 wherein said second ring isfixed to a drive component of a thrust mechanism such that rotation ofsaid drive component results in translational movement of a drivencomponent of said thrust mechanism for controlling the magnitude of saidclutch actuation force applied to said transfer clutch.
 6. The powertransmission device of claim 5 wherein said thrust mechanism is a ballramp unit having a first cam plate as its drive component, a second camplate as its driven component, and rollers engaging a cam surface formedbetween said first and second cam plates, and wherein said cam surfaceis configured to cause translational movement of said second cam platein response to rotary movement of said first cam plate.
 7. The powertransmission device of claim 6 wherein an increase in fluid pressure insaid actuation chambers causes said second ring and said first cam plateto rotate in a first direction relative to said first ring for causingcorresponding translational movement of said second cam plate from afirst position toward a second position relative to said transferclutch, and wherein a decrease in fluid pressure in said actuationchambers causes said second ring and said first cam plate to rotate in asecond direction relative to said reaction ring for causing movement ofsaid second cam plate toward its first position.
 8. The powertransmission device of claim 1 wherein said first rotary member is afirst shaft in a transfer case and said second rotary member is a secondshaft of said transfer case.
 9. The power transmission device of claim 1wherein said first rotary member is driven by a powertrain of a motorvehicle and said second rotary member is connected to a differentialunit of a drive axle assembly.
 10. The power transmission device ofclaim 1 defining a drive axle assembly having a differential unitinterconnecting a pair of axleshafts, and wherein said first rotarymember is a differential carrier of said differential unit, said secondrotary member is one of said axleshafts, and said torque transfermechanism is arranged to adaptively limit slip between said axleshafts.11. A power transfer device for use in a motor vehicle having apowertrain and first and second drivelines, comprising: a first shaftdriven by the powertrain and adapted for connection to the firstdriveline; a second shaft adapted for connection to the seconddriveline; a torque transmission mechanism for transferring drive torquefrom said first shaft to said second shaft, said torque transmissionmechanism including a transfer clutch operably disposed between saidfirst shaft and said second shaft and a clutch actuator for engagingsaid transfer clutch, said clutch actuator includes a rotary operatorand a thrust mechanism, said rotary operator having first and secondcomponents which define an actuation chamber that is adapted to receivepressurized fluid, said first component is fixed for rotation with oneof said first and second shafts and said second component is adapted torotate relative to said first component in response to the pressurizedfluid in said actuation chamber, said thrust mechanism operable forapplying a clutch actuation force to said transfer clutch in response torotation of said second component relative to said first component; anda control system for regulating the pressurized fluid supplied to saidactuation chamber.
 12. The power transfer device of claim 11 whereinangular movement of said second component to a low pressure positionrelative to said first component causes said thrust mechanism to belocated in a first position for causing a minimum clutch engagementforce to be applied to said transfer clutch, wherein angular movement ofsaid second component to a high pressure position relative to said firstcomponent causes said thrust mechanism to move to a second position forcausing a maximum clutch engagement force to be applied to said transferclutch, and wherein said second component is moveable between its lowpressure and high pressure positions due to the magnitude of thepressurized fluid delivered from a pump through a control valve to saidactuation chamber.
 13. The power transfer device of claim 12 whereinsaid first component of said rotary operator is a reaction ring having afirst body segment and a plurality of first lugs which define a seriesof channels therebetween, wherein said second component is an actuatorring having a second body segment and a plurality of second lugs whichextend into said channels so as to define a plurality of said actuationchambers between said first and second lugs, and wherein said actuatorchambers are in fluid communication with an outlet of said control valveand said fluid pump is operable to draw low pressure fluid from a sumpand deliver high pressure fluid to said control valve such thatselective control of said control valve results in rotary movement ofsaid actuator ring relative to said reaction ring.
 14. The powertransfer device of claim 13 wherein said actuator ring is fixed to adrive component of said thrust mechanism such that rotation of saiddrive component results in translational movement of a driven componentof said thrust mechanism for exerting said clutch actuation force onsaid second friction clutch.
 15. The power transfer device of claim 14wherein said thrust mechanism is a ball ramp unit with a first cam ringas its drive component, a second cam ring as its driven component, androllers retained in cam tracks formed between said first and second camrings, and wherein said cam tracks are configured to cause translationalmovement of said second cam plate relative to said transfer clutch inresponse to rotary movement of said first cam plate.
 16. A powertransfer device for use in a motor vehicle having a powertrain and firstand second drivelines, comprising: an input member adapted to receivedrive torque from said powertrain; a first output member adapted toprovide drive torque to the first driveline; a second output memberadapted to provide drive torque to the second driveline; a gearsetoperably interconnecting said input member to said first and secondoutput members; a torque transmission mechanism for limiting speeddifferentiation between said first and second output members, saidtorque transmission mechanism including a transfer clutch operablydisposed between any two of said input member and said first and secondoutput members and a clutch actuator for controlling engagement of saidtransfer clutch, said clutch actuator including a rotary operator and athrust mechanism, said rotary operator having first and secondcomponents defining an actuation chamber therebetween that is adapted toreceive pressurized fluid, said first component is fixed for rotationwith one of said input and output members and said second component isadapted to rotate relative to said first component in response to thepressurized fluid in said actuation chamber, said thrust mechanismoperable for applying a clutch actuation force to said transfer clutchin response to rotation of said second component relative to said firstcomponent; and a hydraulic control system for regulating the pressurizedfluid supplied to said actuation chamber.
 17. The power transfer deviceof claim 16 wherein movement of said second component to a low pressureposition relative to said first component causes said thrust mechanismto be located in a first position for applying a minimum actuation forceto said transfer clutch, wherein movement of said second component to ahigh pressure position relative to said first component causes saidthrust mechanism to move to a second position for applying a maximumactuation force to said transfer clutch, and wherein said secondcomponent is moveable between its low pressure and high pressurepositions due to the magnitude of the pressurized fluid delivered from apump through a control valve to said actuation chamber.
 18. The powertransfer device of claim 17 wherein said first component of said rotaryoperator is a reaction ring having a plurality of first lugs whichdefine a series of channels therebetween, and said second component isan actuator ring having a plurality of second lugs which extend intosaid channels so as to define a plurality of said actuation chambersbetween said first and second lugs, wherein said actuator chambers arein fluid communication with an outlet of said control valve, and whereinsaid fluid pump is operable to draw fluid from a sump and deliver fluidthrough said control valve to said actuation chambers for causing rotarymovement of said actuator ring relative to said reaction ring.
 19. Thepower transfer device of claim 18 wherein said actuator ring is fixed toa drive component of said thrust mechanism such that rotation of saiddrive component results in translational movement of a driven componentof said thrust mechanism for controlling the magnitude of said clutchactuation force applied to said transfer clutch.
 20. The power transferdevice of claim 19 wherein said thrust mechanism is a ball ramp unitwith a first cam ring as its drive component, a second cam ring as itsdriven component, and rollers retained in cam tracks formed between saidfirst and second cam rings, said cam tracks configured to causetranslational movement of said second cam ring in response to rotarymovement of said first cam ring, and wherein such movement of saidsecond cam ring causes said clutch actuation force to be applied to saidtransfer clutch.
 21. The power transfer device of claim 20 wherein anincrease in pressure of the pressurized fluid in said actuation chamberscauses said actuator ring and said first cam ring to rotate in a firstdirection relative to said reaction ring for causing said second camring to axially move from a retracted position toward an extendedposition relative to said transfer clutch, and wherein a decrease inpressure of the pressurized fluid in said actuation chambers causes saidactuator ring and said first cam ring to rotate in a second directionrelative to said reaction ring for causing said second cam ring toaxially move toward its retracted position.
 22. A torque transfermechanism for controlling transfer of drive torque between first andsecond rotary members comprising: a transfer clutch operably disposedbetween the first and second rotary members; a rotary clutch operatorhaving coaxially aligned first and second components defining aplurality of actuation chambers therebetween; and a hydraulic controlsystem operable for controlling fluid pressure delivered to saidactuation chambers so as to control angular movement of said secondcomponent relative to said first component for controlling the magnitudeof a clutch engagement force applied to said transfer clutch.
 23. Thepower transmission device of claim 22 wherein said hydraulic controlsystem includes a pump, a motor driving said pump and a control valvedisposed in a hydraulic circuit between said pump and said actuationchambers for regulating the fluid pressure supplied to said actuationchambers.
 24. The power transmission device of claim 22 wherein angularmovement of said second component to a low pressure position relative tosaid first component causes a minimum clutch actuation force to beapplied to said transfer clutch, wherein angular movement of said secondcomponent to a high pressure position relative to said first componentcauses a maximum clutch actuation force to be applied to said transferclutch, and wherein said second component is moveable between its lowpressure and high pressure positions due to the magnitude of the fluidpressure delivered from said pump through said control valve to saidactuation chambers.
 25. The power transmission device of claim 22wherein said first component of said rotary operator is a reaction ringhaving a body segment and plurality of first lugs so as to define aplurality of channels therebetween, and wherein said second component ofsaid rotary actuator is an actuator ring having a body segment and aplurality of second lugs which extend into said channels so as to definea series of said actuation chambers between adjacent pairs of said firstand second lugs.
 26. The power transmission device of claim 25 whereinsaid actuator ring is fixed to a drive component of a thrust mechanismsuch that rotation of said drive component results in translationalmovement of a driven component of said thrust mechanism for controllingthe magnitude of said clutch actuation force applied to said transferclutch.
 27. The power transmission device of claim 26 wherein saidthrust mechanism is a ball ramp unit having a first cam plate as itsdrive component, a second cam plate as its driven component, and rollersretained in cam tracks formed between said first and second cam plates,and wherein said cam tracks are configured to cause translationalmovement of said second cam plate in response to rotary movement of saidfirst cam plate.
 28. The power transmission device of claim 22 whereinthe first rotary member is a first shaft in a transfer case and thesecond rotary member is a second shaft of said transfer case.
 29. Thepower transmission device of claim 22 wherein the first rotary member isdriven by a powertrain of a motor vehicle and the second rotary memberis connected to a differential unit of a drive axle assembly.